SBS is one of many nonlinear phenomena which can adversely affect system performance in fiber optic systems. Brillouin scattering within a silica optical fiber results from photons being scattered by localized refractive index variations induced by acoustic (i.e., sound) waves. These refractive index variations are caused by acoustic vibrations in the silica lattice that makes up the fiber core. Furthermore, owing to the dependence of refractive index on light intensity in the nonlinear regime, the presence of intense light in the fiber will also induce lattice vibrations which, in turn, induce acoustic waves, that then scatter more light. When the SBS threshold power is exceeded (as low as about 10 mW per channel in some WDM systems), light from an intense forward propagating signal (e.g., the transmitted signal) can provide gain for (i.e., stimulate) a backward propagating signal (known as a Stokes signal). In this fashion, the Stokes signal can degrade the transmitted signal significantly.
Yet many applications require that the transmitted signal be launched at relatively high power, and anything, including SBS, which limits the maximum launch power presents a problem. For example, limiting the launch power reduces the allowable un-repeatered span length in fiber optic transmission systems, as well as the number of splits (or fanouts) which can be utilized in a fiber-based distribution system (e.g., a CATV system). One way to alleviate this problem is to increase the power at which the onset of SBS occurs (i.e., increase the SBS threshold). This threshold is arbitrarily defined as the level of launched optical power at which the power of the backward Stokes signal becomes equal to the power of the Rayleigh scattered signal; i.e., the total reflected power has doubled.
The prior art has devised numerous schemes for increasing the SBS threshold, but none is entirely satisfactory. Most of these schemes rely on the fact that the efficiency for SBS decreases as the linewidth of the transmitted signal source is increased. Consequently, artificially broadening the spectrum of that source via modulation serves to increase the SBS threshold. One approach calls for an external phase modulator to modulate the output of a laser transmitter, thereby broadening the spectrum of the transmitted signal by randomly changing its phase. A second approach utilizes wavelength dithering or detuning. A small specialized heating element is used to change the local laser temperature and thus its wavelength by a small amount. The frequency of the wavelength dither is on the order of a few kilohertz. However, these approaches require complex or high-voltage driving waveforms to broaden the spectrum. Alternatively, small-signal direct modulation of a DFB laser transmitter has also been suggested. But, when relatively large SBS thresholds are required, this approach results in substantial amplitude modulation (AM) which may degrade system performance. Another prior art approach, significantly different in that it does not involve artificially broadening the transmitted signal spectrum, suppresses SBS by applying duobinary modulation to the transmitted signal. Due to the absence of an optical carrier in the spectrum, the SBS threshold is increased compared to a conventional binary format. However, the duobinary format is not suitable for some systems applications.
Thus, a need remains in the art for an SBS reduction technique that can implemented with a simple, low voltage control signal, does not require special coding formats, and has low levels of residual AM.